Hydraulic and pneumatic cylinders have long been used in applications requiring high mechanical forces in locations that lack space for motors or engines capable of generating such forces. Transmitting force by hydraulic fluid or pneumatic gas and a cylinder is a common practice in many industries. With the advent of computer controls in hydraulic systems, such systems are required to perform increasingly challenging functions in a wide variety of industries and applications. As newer, high volume, computer controlled, low cost applications emerge, hydraulic systems are appearing in markets requiring xe2x80x9cmaintenance freexe2x80x9d performance, free of noise, repairs and fluid leaks. Leaks of hydraulic fluid, even drops over the product lifetime, may constitute a hazard. These newer applications require cylinders to evolve from heavy-duty efficient transmitters of extreme forces, or from low volume, low reliability disposable cylinders, to high reliability and low cost cylinders capable of being produced repeatably in large quantities.
Conventional cylinder constructions involve variations on the basic cylinder components: rod, piston with piston seal, rod cap with seals, cylinder tube and cylinder blind end cap. Rod and blind end caps typically are attached to the cylinder tube by threads, welding, retaining rings or crimping. Non-welded cylinders generally require elastomeric seals to contain the hydraulic fluid. Conventional cylinder construction techniques have inherent reliability or cost problems when used in high volume applications. Welded cylinders must be extensively tested prior to shipping. Excessive heat during welding creates a risk of heat damage to specialized seals and other components. Threaded cylinders require extensive machining, need additional machined features to protect seals during assembly, present difficulties in maintaining concentricity between the separate cylinder components, and involve many assembly steps. Ring-retained and crimped cylinders follow basically the same manufacturing and assembly steps as threaded cylinders. They require less machining and provide for easier assembly, but have lower performance limits. Producing cylinders in high volumes with high reliability requires controlling the cylinder design and manufacturing processes to obtain a high degree of product acceptance without depending on final testing.
Therefore, it is an object of the present invention to provide a fluid actuator cylinder design that provides a high degree of reliability at relatively low cost.
Another object is to provide a hydraulic or pneumatic cylinder having the high performance characteristics associated with welded and threadedly attached end caps, while avoiding the high cost and difficulties associated with welded and threaded end caps.
A further object is to provide an improved process for assembling a fluid actuator cylinder.
Yet another object is to provide, in a fluid actuator, a cylindrical enclosure that incorporates an end cap or end closure containment feature that is asymmetrical in the sense of exerting a greater force on the end cap or other member in the axial direction that requires more force, i.e. the direction opposite to the force applied to the end cap by pressurized fluid when the actuator is in use.
To achieve these and other objects, there is provided an actuator enclosure for containing a reciprocating piston. The enclosure includes the cylindrical housing defining a chamber including a first region, a second region opposite the first region and including an open end of the housing, and a medial region between the first and second regions. A first end closure member is insertable in a first axial direction into the chamber through the open end. The first closure member is shaped for a conforming and contiguous surface engagement with the housing at a first predetermined location along the first region. A second end closure member also is insertable in the first axial direction into the chamber through the open end. The second end closure member is shaped for a conforming and contiguous surface engagement with the housing at a second predetermined location along the second region. A closure member containing structure, integral with the housing, is positioned to engage the first closure member substantially upon a complete insertion thereof to the first predetermined location, thereby to prevent further travel of the first closure member in the first axial direction. The housing is adapted to accommodate a piston for reciprocation along the medial region of the chamber. A selected one of the first and second closure members includes an opening adapted to slideably support a piston rod coupled to the piston.
The preferred closure member containing structure is an annular rim welded to an edge of the housing adjacent the first region, and extended radially inwardly from the housing. The rim thus acts as a stop, preventing the end cap or other closure member from moving any further in the first axial direction after it encounters the rim. Because the rim is welded to the cylindrical housing, it provides the retaining strength of a welded end cap. As a result the actuator enclosure is usable in applications requiring, among conventional cylinders, either threaded or welded end caps. At the same time, the extensive machining required of threaded cylinders and end caps is avoided.
With respect to conventional welded cylinders, a considerable advantage arises from the fact that the annular rim can be welded to the cylindrical housing before insertion of the end cap or other closure member. Consequently, there is no risk of heat damage to specialized seals or other internal cylindrical components during welding.
A closure member mounting device, e.g. a retaining ring, can be used to releasably secure the end cap against movement in the second, opposite axial direction away from its predetermined location. More particularly, a portion of the end closure member can extend beyond the first end region of the housing, in which case the retaining structure can include a groove formed circumferentially about the closure member and a retaining ring removably mounted within the groove. The retaining ring is relatively weak compared to the welded annular rim, exerting considerably less force upon the closure member. However, force in opposition to the retaining ring, caused primarily by friction of the piston rod during retraction when the closure member provides the rod cap, is considerably less than the force of pressurized fluid against the end cap when the piston rod is extended.
The invention affords a xe2x80x9chybridxe2x80x9d construction technique combining the strength of welded cylinders, the sealing reliability of elastomeric seals, and the assembly ease of ring-retained cylinders. In one version, the rod end cap of the cylinder is retained in the cylinder tube by a welded ring. This welded ring gives the cylinder the strength of a welded cylinder for withstanding maximum operating pressures while eliminating the need to rely on the weld as a hydraulic seal. The ring can be welded onto the cylinder tube prior to assembly, eliminating heat damage to the seals and other internal cylinder components during welding. The cylinder tube/ring combination can be painted or plated prior to assembly for corrosion resistance without the special handling required for a completed cylinder.
The blind end of the cylinder preferably is flared to allow convenient assembly of all cylinder components through the cylinder blind end. This flaring eliminates sharp or abrupt edges that can damage a seal during assembly. The rod cap and seals, the rod, the piston and piston seals are all assembled through the flared end of the cylinder. Consequently, there is virtually no chance of seal damage during cylinder assembly. Reliability and performance are enhanced, because concentricity between the rod and piston bearing surfaces in the rod end cap and tube are aligned during assembly by the cylinder tube itself, and can be completely controlled to very close tolerances by CNC (computer numerical controlled) machining operations in manufacturing the separate component parts. There is no need to rely on assembly techniques or fixturing to maintain proper alignments. This eliminates cylinder binding. Cylinder performance can be controlled by statistical control or other process control techniques during manufacturing of the separate cylinder components. By transferring the controlling factors to the component manufacturing level, reliability is improved. Manufacturing becomes easier because it is more controllable, and assembly can be rapid and repeatable. The resulting cylinders are far more economical and reliable.
Preferably the first end closure member provides the rod end cap, and the second end closure member provides the blind end cap. The blind end cap, accommodated in the flared end of the preferred housing, is larger in diameter than the rod end cap. The blind end cap does not require an axial opening therethrough to accommodate the piston rod. Accordingly, a transverse opening can be formed through the blind end cap to accommodate a pin used to support the actuator and at the same time releasably mount the blind end cap within the housing, specifically by a simultaneous extension of the pin through the blind end cap opening and two openings through the housing, on opposite sides of the housing that align with the blind end cap opening when the end cap is at its predetermined location. The pin can be secured by two bushings, one inserted through each of the housing openings into the end cap opening. As with the rod, piston and rod cap, the blind end cap has a seal located inwardly of the flare to ensure maximum seal integrity. Ultimate cylinder strength, rating and safety factors become functions of the blind end attachment and the parameters under which the cylinder is used in each application.
Another aspect of the present invention is a process for assembling a fluid actuator, comprising the following steps:
a. providing a cylinder having first and second opposite open ends, a first diameter over a majority of its length including the first end, a second diameter larger than the first diameter along an end region of the cylinder including the second end, and a transition region providing a gradual transition between the first diameter and the second diameter;
b. securing an end cap containment feature with respect to the first end of the cylinder;
c. inserting a first end cap into the cylinder through the second end, and moving the first end cap in a first axial direction along the cylinder until it contacts the containment feature and substantially closes the first end upon reaching a first predetermined location within the cylinder;
d. after so inserting the first end cap, inserting a piston into the cylinder through the second end, and moving the piston in the first axial direction along the cylinder to a location beyond the transition region;
e. after so inserting the piston, inserting a second end cap into the cylinder through the second end to a second predetermined location to substantially close the second end; and
f. extending the piston rod from the piston, through an opening provided through a selected one of the end caps, to a piston rod termination outside of the cylinder.